As background, the advantages of an environmental scanning electron microscope over the standard scanning electron microscope ("SEM") lie in its ability to produce high-resolution electron images of moist or non-conductive specimens (e.g., biological materials, plastics, ceramics, fibers) which are extremely difficult to image in the usual vacuum environment of the SEM. The environmental scanning electron microscope allows the specimen to be maintained in its "natural" state, without subjecting it to the distortions caused by drying, freezing, or vacuum coating normally required for high-vacuum electron beam observation. Also, the relatively high gas pressure easily tolerated in the ESEM specimen chamber acts effectively to dissipate the surface charge that would normally build up on a non-conductive specimen, blocking high quality image acquisition. The environmental scanning electron microscope also permits direct, real-time observation of liquid transport, chemical reaction, solution, hydration, crystallization, and other processes occurring at relatively high vapor pressures, far above those that can be permitted in the normal SEM specimen chamber.
Typically, in an ESEM, the electron beam is emitted by an electron gun and passes through an electron optical column with an objective lens assembly having a final pressure limiting aperture at its lower end thereof. In the electron optical column, the electron beam passes through magnetic lenses which are used to focus the beam and direct the electron beam through the final pressure limiting aperture.
The beam is subsequently directed into a specimen chamber through the final pressure limiting aperture wherein it impinges upon a specimen supported upon the specimen stage. The specimen stage is positioned for supporting the specimen approximately 1 to 25 mm below the final pressure limiting aperture so as to allow the beam of electrons to interact with the specimen. The specimen chamber is disposed below the optical vacuum column and is capable of maintaining the sample enveloped in gas, typically water vapor, at a pressure of approximately between 10.sup.-2 and 50 Torr in registration with the final pressure limiting aperture such that a surface of the specimen may be exposed to the charged particle beam emitted from the electron gun and directed through the final pressure limiting aperture.
As stated in U.S. Pat. No. 4,992,662, the original concept of an environmental scanning electron microscope, as suggested in U.S. Pat. No. 4,596,928, was to maintain the specimen chamber in a gaseous environment such that the gaseous environment acted as a conditioning medium in order to maintain the specimen in a liquid or natural state. In addition, the utilization of the gaseous environment of the specimen chamber as a medium for amplification of the secondary electron signals is described in U.S. Pat. No. 4,785,182.
In the environmental SEM of U.S. Pat. No. 4,823,006, electron beam observation of unprepared, full-sized specimens at high vacuum pressure was made possible due to the combination of pressure control and signal detection means, housed entirely within the magnetic objective lens of the electron beam column. The environmental SEM design of U.S. Pat. No. 4,823,006 satisfied the simultaneous requirements for pressure control, electron beam focusing, and signal amplification, while providing no practical limitations on specimen handling or microscopic resolving power.
U.S. Pat. No. 4,880,976 describes the design and need for a gaseous secondary electron detector for an ESEM. Subsequent prior art describe improved secondary electron detectors and detectors that detect backscattered electrons, such as in U.S. Pat. No. 4,897,545.
However, it has been found desirable to provide a dedicated gaseous detector that is intended to collect only a backscattered electron signal. Moreover, it has been found desirable to provide a dual electron detector that can be switched between the secondary and backscattered electron detection modes.
Many different types of signals are generated in a conventional scanning electron microscope ("SEMI"), when the primary electron beam strikes the sample. The two most important electron signals are:
a) Secondary electrons ("SE") which produce the highest resolution images which show the topography of the surface of the sample; and PA1 b) Backscattered electrons ("BSE") which produce a lower resolution image but the signal is very sensitive to changes in the density of the sample material. The BSE images are also often used to show the distribution of different material components of the sample.
The conventional high vacuum SEM has an SE detector as standard and most users also purchase a separate BSE detector.
In addition, a fundamental aspect of an ESEM detector is the amplification of the electron signal in the gaseous environment of the specimen chamber. This is important because the electron signal levels used in an SEM are normally too small to be directly connected to an amplifier. The noise from the amplifier would be too high to make the SEM a practical instrument. In the conventional high vacuum SEM, the secondary electron signal is amplified (with negligible added noise) by a photomultiplier as part of a complex arrangement originally described by Everhart and Thornley. Hence, this type of detector is commonly called the Everhart-Thornley (E-T) detector. The E-T detector will not function in the ESEM because the high voltages used will discharge in the gas environment of the ESEM.
Hence, it is extremely desirable to provide a gaseous detector used in an ESEM which is designed to cause amplification of the signals to a high enough level to make the noise of the following electronics low.
Signal amplification in the gaseous environment of an ESEM is schematically represented in FIG. 1. As shown therein, an environmental scanning electron microscope provides a device for generating, amplifying and detecting secondary and backscattered electrons emanating from a surface of a sample being examined. A beam of electrons 10 is emitted through an electron optical column of an objective lens assembly 11 by an electron gun (not shown). The vacuum optical column includes a final limiting pressure aperture 14 at its lower end thereof. A beam 10 is directed into a specimen chamber 16 wherein it impinges upon a specimen 18 supported on a specimen stage 20. The specimen mount or stage 20 is located within the specimen chamber 16 and is positioned for supporting specimen 18 approximately 1 to 25 mm, and preferably 1 to 10 mm, below final pressure limiting aperture 14 so as to allow the beam of electrons to interact with the specimen. The specimen chamber is disposed below the optical vacuum column and is capable of maintaining the sample 18 enveloped in gas, preferably nitrogen or water vapor, at a pressure of approximately between 10.sup.-2 and 50 Torr in registration with the pressure limiting aperture such that a surface of the specimen may be exposed to the charged particle beam emitted from the electron gum and emitted through the pressure limiting aperture 14.
The ESEM detectors use an electric field in the gas to amplify an electron signal. When the primary beam 10 strikes the specimen 18, electrons are liberated. The electron field between the sample 18 and the detector electrode 22, held at a positive voltage, accelerates a signal electron, as at 24 until it has enough energy to ionize a gas molecule which also liberates another electron, as represented by reference numeral 27 in FIG. 1. The two electrons will be further accelerated to thereby generate more electrons, as at 28. This process can generate enough amplification for the electron current to be passed directly to a low noise amplifier 30. The amplification is typically in the range of 100 to 2,000. The amplification principle applies to any electron that is in the gas. FIG. 1 illustrates amplification of the low energy "secondary electrons" generated at the surface of the specimen.
Electrons can also be generated in the gas by backscattered electrons ("BSE"). These are high-energy electrons from the primary beam that are reflected from the sample. The BSEs have a high velocity and this high velocity reduces the chance that the BSE will strike a molecule in the gas between the sample and the detector. Hence, it has been found that only a small fraction of the BSE will generate a useful gas interaction. Accordingly, most of the signals collected by the detector electrode are generated by amplification of the secondary electrons.
U.S. Pat. No. 5,362,964 describes improvements in the design of a gaseous detector for an ESEM to maximize the SE collection while minimizing the collection of signals generated by other sources such as BSE. It has therefore been found desirable to provide a detector configuration for an ESEM that is designed to collect only signals generated by the BSE.
U.S. Pat. No. 4,897,545 to Danilatos describes a multi-electrode structure wherein the different electrodes will collect different proportions of SE and BSE information. Some electrodes collect a signal that is rich in SE, and some collect a signal that is rich in BSE. The '545 patent, however, does not relate to an electrode detector that collects only the BSE signal. Moreover, prior art exists which converts backscattered electrons (BSE) into secondary electrons (SE) and then collects the resulting SE signal--but only in high-vacuum SEMs. However, utilization of this conversion principle to generate a BSE signal detector in a gaseous environment does not exist.